Lets build a wiki, from everything mentioned:
https://docs.google.com/d
***
http://gen.lib.rus.ec/ (Library Genesis)
http://sci-hub.cc/ (for papers)
http://www.moreofit.com/s
http://www.calvinshub.com
*** Dead links
kat.ph: seems dead.
http://bookova.com (dead)
http://btdigg.org (dead)
http://btdb.in/ is a search engine for bittorrent distributed hash table, being sort of a meta-search engine covering a lot of stuff that's not indexed by Google or larger torrent trackers like Pirate Bay or Demonoid. A lot of books can be found in huge e-book bundles.
Edit (August 2016): replaced btdigg with btdb, as btdigg seems to be down for more than a month or two, and btdb came in its place.
Best Alternatives to http://Library.nu :
1. http://bookzz.org/ or http://en.bookfi.org/
6. http://it-ebooks.info/ – Lots of Computer Science subject related ebooks.
8. http://www.allitebooks.co
9. http://mirtitles.org/ – Russian Website
10. http://www.ebookl.com/ or https://www.hadoop.so/
12. http://www.freebookspot.e
16. https://www.freeetextbook
17. http://www.textbooknova.c
Source : Library.nu Alternatives | Websites For Downloading Unlimited Free eBooks
You can refer the source article for more websites which are even legal for downloading free ebooks.